Buyers' guide: Radiant floor heating and cooling systems

Radiant floor heating has been used for centuries. The Romans channeled hot air underthe floors of their villas. The Koreans channeled hot flue gases under their floors beforeventing them up the chimney.

In the 1930s, architect Frank Lloyd Wright piped hot water through the floors of manyof his buildings. Some home builders surveys have shown that, if given a choice,most new home owners prefer radiant floor heat over other types of systems.

Advantages of Radiant Floor Heating

Most people who own radiant floor heating feel that the most important advantages arecomfort and quiet operation. Radiant floor systems allow even heating throughout the wholefloor, not just in localized spots as with wood stoves, hot air systems, and other typesof radiators. The room heats from the bottom up, warming the feet and body first. Radiantfloor heating also eliminates the draft and dust problems associated with forced-airheating systems.

Even heat distribution equates with lower heating bills. With radiant floor heating,you can set the thermostat several degrees lower. This is because the entire surface ofthe floor radiates about the same amount of heat that the human body does, making theoccupant feel warm even though the air temperature might be only 65 o F (18o C). It also radiates this heat for a longer period of time than a forced airsystem.

Another advantage is that radiant systems do not increase the infiltration of outsideair into the house structure as forced air systems generally do. Radiant floor heatingalso allows lower boiler temperatures allowing them to last longer (a 45 year life is notunusual). Radiant floors operate between 85-140o F (29-60o C),compared to other heating systems range of 130-160o F (54-71oC). Fuel saving of 15% to 20% over a forced air system is common.

To some, the greatest advantage of radiant floor heating is aesthetic. The system is"invisible." There are no heat registers or radiators to obstruct furniturearrangements and interior design plans. Radiant floor systems also eliminate the fan noiseof forced hot air systems.

Types of Radiant Floor Heating

There are three types of radiant floor heat:

radiant air floors (air is the heat carrying medium);

electric radiant floors;

and hot water (hydronic) radiant floors.

All three types can be further subdivided by the type of installation: those that makeuse of the large thermal mass of a concrete slab floor or lightweight concrete over awooden subfloor (these are called "wet" installations); and those in which theinstaller "sandwiches" the radiant floor tubing between two layers of plywood orattaches the tubing under the finished or subfloor (dry installations).

Because air cannot hold large amounts of heat, radiant air floors are notcost-effective in residential applications, and are seldom installed.

Electric radiant floors are usually only cost-effective if your electric utilitycompany offers time-of-use rates. Time-of-use rates allow you to "charge" theconcrete floor with heat during off-peak hours (approximately 9 pm to 6 am). If the floorsthermal mass is large enough, the heat stored in it will keep the house comfortable foreight to ten hours, without any further electrical input. This saves a considerable numberof energy dollars compared to heating at peak electric rates during the day.

Hydronic (liquid) systems are the most popular and cost-effective systems forheating-dominated climates. They have been in extensive use in Europe for decades.Hydronic radiant floor systems pump heated water from a boiler through tubing laid in apattern underneath the floor. The temperature in each room is controlled by regulating theflow of hot water through each tubing loop. This is done by a system of zoning valves orpumps and thermostats.

Wet installations are the oldest form of modern radiant floor systems. In a"wet" installation, the tubing is embedded in the concrete foundation slab, orin a lightweight concrete slab on top of a subfloor, or over a previously poured slab. Ifthe new floor is not on solid earth, additional floor support may be necessary because ofthe added weight. You should consult a professional engineer to determine the floorscarrying capacity.

However, due to recent innovations in floor technology, "dry" floors havebeen gaining a lot of popularity over wet floors. Much of this is because a dry floor isfaster and less expensive to build. There are several ways to make a dry radiant floor.Some "dry" installations involve suspending the tubing underneath the subfloorbetween the joists. This method usually requires drilling through the floor joists inorder to install the tubing.

Reflective insulation must also be installed under the tubes to direct the heat upward.Tubing may also be installed from above the floor, between two layers of subfloor. Inthese instances, the tubes are often in aluminum diffusers that spread the watersheat across the floor in order to heat the floor more evenly. The tubing and heatdiffusers are secured between furring strips (sleepers) which carry the weight of the newsubfloor and finished floor surface.

At least one company has improved on this idea by making a plywood subfloor materialmanufactured with tubing grooves and aluminum heat diffuser plates built into them. Themanufacturer claims that this product makes a radiant floor system (for new construction)considerably less expensive to install and faster to react to room temperature changes.Such products also allow for the use of half as much tubing since the heat transfercharacteristics of the floor is greatly improved over more traditional dry or wet floors.

Floor Coverings

Although ceramic tile is the most common floor covering for radiant floor heating,almost any floor covering can be used. However, some perform better than others. Commonfloor coverings like vinyl and linoleum sheet goods, carpeting, wood or bare concrete isoften specified. However, it is wise to always remember that anything that can insulatethe floor also reduces or slows the heat entering the space from the floor system. This inturn increases fuel consumption.

If you want carpeting, use a thin carpet with dense padding and install as littlecarpeting as possible. If some rooms, but not all, will have a floor covering then thoserooms should have a separate tubing loop to make the system heat these spaces moreefficiently. This is because the water flowing under the covered floor will need to behotter to compensate for the floor covering.

Most radiant floor references also recommend using laminated wood flooring instead ofsolid wood. This reduces the possibility of the wood shrinking and cracking from thedrying effects of the heat. While solid wood flooring can be used, the installer isstrongly advised to be very familiar with radiant floor systems before attemptingto install natural wood flooring over a radiant floor system. Most manufacturers andmanuals relating to radiant floors offer guidelines to help you resolve these issues.

Types of Tubing

Older radiant floor systems used either copper or steel tubing embedded in the concretefloors. Unless the builder coated the tubing with a protective compound, a chemicalreaction between the metal and the concrete often led to corrosion of the tubing, and toeventual leaks. Major manufacturers of hydronic radiant floor systems now use cross-linkedpolyethylene (PEX) or rubber tubing with an oxygen diffusion barrier. These materials haveproven themselves to be more reliable than the older choices in tubing. Fluid additivesalso help protect the system from corrosion.

There have been recent reports of problems with rubber tubing produced by onechemical manufacturer. Leaks develop at the metal connections or fittings, and in somecases the tubing becomes rigid and brittle. It is still not clear what causes thisproblem, but theoretically excessively high water temperatures may be to blame. Tighteningconnections and clamps only temporarily fixes the leaks. Remember this problem onlyconcerns a specific brand of rubber tubing. It does not have anything to do withthe PEX tubing, which has performed very reliably for many decades.

Since the price of copper tubing is considerably lower now than several years ago, itis again gaining some popularity because of its superior heat transfer abilitiesover plastic-based tubing.

Controlling the System

A radiant floor that uses a concrete slab takes many hours to heat up if it is allowedto become cold. This can be very inconvenient while waiting for the slab heat up so it canheat the space. Because of this, most radiant floor systems are not permitted to go into avery deep night setback. Depending on how the floor is constructed, the time it takes tore-heat the floor is sometimes longer than the occupants sleep period.

Many floor systems are also controlled by a floor thermostat instead of a wallthermostat. The system is also often designed to keep the circulation pump(s) runningwhile the thermostat only controls the boilers burner. Other, more sophisticated,types of controls sense the floor temperature, outdoor temperature, and room temperatureto keep the home comfortable. Such a system may also use less fuel.

Although radiant floor systems are usually heated by a boiler, they can also be heatedwith a geothermal heat pump. Such a system offers even greater energy savings in climateswhere the heating and cooling loads are similar in size. Another alternative for smallhouses, or those with small heating loads, is to use an ordinary gas water heater tosupply the radiant floor system.

Radiant Floor Cooling

Radiant floor tubing can also be used to cool a house, but presently it is onlyappropriate for dry climates. The floor temperature is held at 68o F (20oC) by using either a small cooling machine (chiller) connected to the floor tubing or thesteady 55o F (13 o C) temperature of the ground by means of an earthloop. In arid climates, the cool floor can be used to supplement or replace standardducted air systems. However, in humid climates, problems with over-cooling the floor couldlead to wet slippery surfaces and fungus growth. Radiant floor cooling technology is stillin the experimental stages in most areas, but is rapidly gaining popularity in Europewhere cooling needs are generally small.

Cost of Radiant Floor Heating

The cost of installing a hydronic radiant floor is approximately $4.00 to $6.00 persquare foot ($40-$60 per square meter). This fluctuates depending on the size of the room,the type of installation, the floor covering, remoteness of the site, and the cost oflabor.